It is known in x-ray radiography and x-ray tomography to determine the absorption coefficient μ (x, y, z) of an object in a precisely spatially resolved fashion, and to prepare an image of the object on the basis of this information. This imaging technique is based on the so-called absorption contrast. It is widely applied in medical diagnostics and in nondestructive testing in industry.
In the case of absorption contrast imaging, the various parts of the objects are weighted according to their mass absorption coefficient. A rough classification of the element concentration or of the tissue type of the object can be specified by evaluating the local absorption coefficient μ (x, y, z). In years gone by, the spatial resolution of this imaging has risen continuously and in the meantime pressed forward into the micrometer range.
It is known in x-ray radiography and x-ray tomography to alternately vary the tube voltage of a focus detector system during the scan, or to use focus detector systems arranged in an offset fashion and having different energy spectra, and thus to scan an object simultaneously with different radiation energies and to obtain projections with a set composed of dual energy data. A reconstruction based thereon then produces a base material decomposition in order to obtain pairs of images with material of high and low Z-value such as, for example, “bone” and “soft tissue”.
This method enables an improved insight into the structure of an examination object, and is also of assistance, for example, in such applications as the bone densitometry of patients. It has also been proposed to extend this dual energy technique to a multiple energy technique that specifies the local absorption coefficient μ (x, y, z) for a number of photoenergies, and permits a finer differentiation. However, it is improbable that it actually achieves a spectral resolution as far as the separation of individual elements.
In addition to absorption, refraction itself is also suitable for x-ray imaging. In the case of so-called phase contrast imaging, the decrement δ of the complex refractive index n=1−δ−iβ is determined in a spatially resolved fashion and reconstructed onto an image. In the case of phase contrast imaging, the various parts of the object are weighted using the gradient of that decrement δ in a fashion emphasizing the contours of the object. Various approaches to specifying phase contrast imaging experimentally have been undertaken in the past 40 years.
Various analytical methods have also been developed in materials analysis. These are, inter alia, x-ray fluorescence (XRF) analysis, electron beam microanalysis (EBMA), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS), infrared spectroscopy (IR), nuclear magnetic resonance (NMR) spectrometry, Raman spectroscopy (RS), x-ray diffraction (XRD) analysis, electron diffraction etc. Many of these methods have been developed in relation to local probes and spatially resolved analysis methods, and this can be used for scanning and/or imaging the objects, and thereby for preparing an image of the elemental distribution, of the distribution of molecular groups or compounds, of the distribution of crystalline phases or of the distribution of physical material properties of the object surface.
In most cases, however, 3D analyses are hampered by the fact that either the information depths are too small, or appropriate optics for imaging element-specific signals are not available. The latter holds especially for signals with a large penetration depth such as x-radiation and gamma radiation.
There is therefore the continuing problem of finding a method and a measuring arrangement with the aid of which it is possible to determine the elemental and/or molecular distribution in the interior of an examination object in a nondestructive fashion.